As a colorant for ink for ink jet printer there has heretofore been mainly used a dye. In recent years, the use of pigments having excellent fastness has been studied. Recorded matters obtained with a pigment as a colorant are superior to that obtained with a dye in respect to fastness such as water fastness and light fastness.
On the other hand, unlike a dye, a pigment is not dissolved in water. Therefore, many studies have been made of the use of, as a colorant for aqueous ink, a pigment in the form of dispersion of particles having a dispersant such as resin adsorbed thereto (hereinafter, this kind of pigment will be referred to as “dispersant type pigment”). For example, JP-A-2000-290578 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a process for the preparation of an aqueous pigment dispersion which comprises a step of dry-grinding a pigment containing coarse particles in the presence of a water-soluble resin, and a step of adding a nonionic surface active agent containing acetylene group to the dry-ground material obtained at the dry-grinding step so that it is dispersed in water. Further, JP-A-2000-351912 discloses a process for the preparation of a pigment composition which comprises dry-grinding a pigment containing coarse particles in the presence of a nonionic surface active agent containing acetylene group and a water-soluble resin or dry-grinding a pigment containing coarse particles in the presence of a nonionic surface active agent containing acetylene group, adding a water-soluble resin to the ground material, and then further dry-grinding the material.
However, even the methods disclosed in the above cited JP-A-2000-290578 and JP-A-2000-351912 are disadvantageous in that the dispersant type pigment can be difficultly kept dispersed stably in an aqueous medium. Thus, the dispersant is separated from the surface of the pigment, causing the pigment particles to be agglomerated to each other or sedimented. Further, if the particle diameter is relatively great, the pigment particles undergoes spontaneous sedimentation due to its weight. Thus, the colorant stays dispersed in particulate form, causing troubles.
In general, an ink for ink jet printer comprises a colorant and water as well as various solvents and surface active agents. Therefore, the conventional method involving the dispersion of a dispersant type pigment in an aqueous solvent can difficultly keep the pigment particles stably dispersed in particulate form. In particular, when the ink comprises a penetrating agent providing a strong penetrating effect incorporated therein to enhance the rate at which it dries on the recording medium, the separation of the dispersant from the surface of the particulate pigment can be possibly accelerated, further deteriorating dispersion stability.
Under the circumstances, various techniques which comprises rendering a pigment itself water-dispersible have been proposed. For example, JP-A-10-195360and JP-A-10-330665 propose a self-dispersible carbon black having a hydrophilic group such as carboxyl group, carbonyl group, sulfone group and hydroxyl group bonded to the surface thereof directly or with the interposition of a polyvalent group. Further, JP-A-8-3498, JP-A-10-195331, and JP-A-10-237349 propose that carbon black be subjected to surface treatment for the improvement of dispersibility thereof. Moreover, JP-A-8-283598, JP-A-10-110110, and JP-A-10-110111 propose a surface-treated pigment having a sulfone group introduced onto the surface of an organic pigment.
Further, JP-A-11-49974 discloses that an organic pigment mass which can be positively charged on the surface thereof is prepared by treating an organic pigment mass having a sulfonic acid group introduced therein with a monovalent metal ion. Further, an aqueous ink composition having an excellent storage stability comprising a particulate pigment prepared from this surface-positively charged organic pigment mass, a dispersant and water is disclosed.
JP-A-2000-53902 proposes an ink comprising as a colorant a particulate pigment (macromolecular chromophore) having a water-solubilizing functional group and a polymer provided on the surface thereof with the interposition of benzene ring introduced onto carbon black.
An ink for ink jet printer is required to exhibit various physical properties. It is particularly important to secure the ink itself with desired storage stability and secure desired ejectability (prevention of clogging, deflected flying, etc.) during ink jet recording. The ink is also required to provide excellent print quality (high print density, printed image having little feathering and bleeding, etc.) with respect to recording paper. In other words, as an ink for ink jet printer, such an ink is required to satisfy all the requirements for ink physical properties, ejectability and print quality at the same time.
Since the foregoing self-dispersible surface-treated pigment has a dispersible group chemically bonded to the surface thereof, the dispersible group cannot be separated therefrom even if the foregoing penetrating agent is incorporated in the ink. Therefore, the foregoing self-dispersible surface-treated pigment is excellent in storage stability such as dispersion stability and thus can easily meet various requirements for ink for ink jet printer.
However, an ink jet printer which can print a high precision image at a high speed has been recently desired. Thus, the ink jet printer has been required to eject an extremely fine ink dot at a higher frequency. Accordingly, in order to secure stabilized print properties, the ink used must meet further requirements.
Unlike the ink comprising a dye, the pigment ink having particles dispersed therein, even if it is a self-dispersible pigment ink having excellent basic properties, can difficultly meet both the requirements for stabilized ejectability and storage stability. Further, even if the pigment is a self-dispersible pigment, the colorant still stays dispersed. Accordingly, the problem that sedimentation occurs when the diameter of dispersed particles is great remains unsolved.
Further, the arrangement comprising a foregoing surface-treated pigment alone is disadvantageous in that the pigment has no fixing components provided on the surface thereof, providing insufficient image fixability. In order to cope with this problem, JP-A-10-110129 discloses that a water-soluble resin is added to improve fixability.
However, the ink composition disclosed in JP-A-10-110129 is disadvantageous in that when printed on a gloss medium requiring an image quality which is equal to or higher than that of color photograph (e.g., gloss paper, gloss film), it can difficultly penetrate into the recording medium. As a result, the liquid component in the ink can little penetrate into the recording medium, leaving the pigment particles behind on the surface of the recording medium and hence impairing the smoothness and gloss of the surface of the image. Thus, the print quality is impaired. Further, the resulting image has an insufficient fixability. Moreover, the foregoing ink is disadvantageous in that it causes bleeding on the area where two or more color inks are imposed on each other or come into contact with each other during full color printing.